I wear a 38 F. I am 5'6" about 195lbs. I need to have a right mastectomy. Can I have a implant that will keep me a 38 F?

It is not possible to make you size 38F after mastectomy & reconstruction, First because there is not enough skin & second there is no implant that can safely give you 38F.

Unfortunately, it is unlikely that you will be able to maintain your
current cup-size following your mastectomy and reconstruction. It is best to
see a trusted and high-recommended board-certified plastic surgeon in your area
who specializes in reconstruction, they should be able to give you an
understanding of what you will be facing, while also providing you with
realistic goals that can meet your aesthetic desires.  

Typically that size is to large to yield a good result with an implant.  If your BMI is over 34 I typically follow the route of using your own tissue to help provide you with a better result.

Unfortunately, it is difficult to give you a definite answer.  Cup size is very variable and there is little consistency with measurements.  The maximum size silicone gel implant available is 800 cc (if your surgeon is not participating in a study looking at larger sized implants).  We do recognize that some patients would benefit from larger implants, but the FDA has not approved them yet.  Standard saline implants can go a bit larger.  Implant volume does not correlate with cup size, so an 800 cc implant would provide a smaller cup size in a larger woman and a larger cup size in a smaller woman.  Height and weight give us an idea of patient size, but it is really the chest wall dimensions that will impact cup size.  See a plastic surgeon in your area that specializes in breast reconstruction.  They should be able to tell you after an exam if your breast volume can be matched with an implant.  Breast reconstruction is a complicated topic, and although the volume may be matched, the shape may likely end up different than your other breast if you have unilateral reconstruction.  Hope this helps.